This present invention relates generally to sorting of waste and recyclable materials using optical techniques. More specifically, this invention relates to the sorting of glass cullet and similar items by measuring the attenuation of light radiated through the cullet.
In conjunction with a continuing worldwide need to preserve natural resources and reduce dependence on landfills and similar waste storage facilities, machines and methods have been devised for automated identification and sorting of waste materials. Among the waste materials of interest is glass cullet, e.g., small pieces of glass of varying characteristics that are distinguished by color. For example, a typical collection of glass cullet may include pieces of glass having green, red, and blue color components or combinations thereof. Prior art glass sorting machines function by sliding the pieces of cullet down what is commonly called a "wearcover." At one or more locations along the wearcover, the cullet will slide under one or more light sources and over one or more light sensors or light receivers arranged to define a sensing area. The pieces of glass cullet having different color characteristics will attenuate the light emitted from the light sources in different amounts. For instance, if a piece of red glass passes between red and green light sources and the light sensors, the green light, as measured by the light sensors, will be attenuated more than the red light.
Most prior art glass sorters have, in fact, employed optical techniques relying primarily on red and green LED light sources. The primary reason for this is that red and green LED's and sensors were the only colors readily available at economically feasible prices. Unfortunately, the use of only red and green light sources in glass sorting restricts the ability of the machine to accurately identify glass containing other color components. This has resulted in the inability of prior art glass sorter to reliably distinguish cullet having measurable level of a blue component. "Blues" (cullet containing a measurable level of a blue color component) are either discarded as waste along with other non-distinguishable impurities, or were mis-sorted in with another color if the "blue" contained a relatively small blue color component. This mis-sorting results in a less pure, lower quality sort, which leads to a lower quality recycled product. Thus, the economic value of the sorted lot, as well as the quality the final product, is lower due to mis-sorts.
Another drawback with prior art glass sorting machines is that a film of crud or other impurities slowly builds up on the wearcover, thereby blocking the light sensors. This buildup, overtime, attenuates all light wavelengths to a measurable level. The film buildup is a by-product of the dirt, sand, water and other material that the cullet sit in, or are exposed to, prior to sorting. Since the cullet are generally trash to be recycled, it is, generally, not cost effective to pre-clean the cullet.
Edge refraction and impurity adhesion also cause glass mis-sorts. Cullet are generally relatively small broken pieces of glass with edges facing a variety of different directions. Light is refracted off at different angles from the different edge angles. The edges also create a prism effect. Because the light is re-directed at different angles, the edges will appear opaque to a sensor. Since cullet are typically one-half inch to two and one-half inch across with edges typically one-sixteenth to one-quarter inch deep, the amount of light refracted, relative to the amount of light passing through the cullet, is not insignificant. This can lead to incorrect color selection and sorting of the cullet or rejection of cullet as foreign matter.
A further problem associated with film buildup on the wearcover of a prior art glass sorter is a shoveling or dozing effect created by cullet in the built-up film layer. This shoveling leaves furrows and other non-uniformities in the film layer. Prior art machines have attempted to compensate for this by slowly adjusting or re-normalizing a baseline light sensor reading, or amplitude value, over time. This has been less than satisfactory due to the non-uniformities previously discussed. Other prior art glass sorting techniques have tried to address this problem by attempting to clean the wearcover and by replacing the wearcover when the film build-up is excessive. However, cleansers have been ineffective and often leave a residue. Frequent replacement of the wearcover is expensive and leads to excessive down time of the sorting unit.
Thus, to increase the economic viability of recycling in this era of limited resources, more accurate glass sorting machines are needed. In the areas where landfill space is still relatively cheap, reducing recycle costs is perhaps even more important since the need for a landfill alternative is not as great. Additionally, increasing the quality of the final sorted product and reducing the product's cost will help shift the market from virgin raw material to post consumer material. This will reduce the need to consume the earth's limited resources. What is needed, then, is an efficient and economical method for sorting cullet by color, including the ability to distinguish blue color components. A means to compensate for non-uniform film build up over the light sensors is also needed, as is an ability to detect and correct errors due to edge refraction. An ability to distinguish ceramics from other opaque objects would be useful. Another useful capability would be to distinguish glass transparent in the visible spectrum from glass opaque in the infrared spectrum. Such "IR--opaque" glass has a different economic value than "IR--transparent" glass.